Method and system for cuff pressure reversions

ABSTRACT

A method and system for determining when to make a reversion to smaller cuff pressure steps during an oscillometric blood pressure measurement is disclosed. The method and system comprise comparing conformance of oscillometric envelope blood pressure data with previous blood pressure data, including measuring a shift between the oscillometric envelope blood pressure data and an oscillometric envelope derived from the previous blood pressure data. In addition, the method and system include making a reversion decision based on whether the shift exceeds an allowable threshold. Once a reversion decision is made a subsequent decision may be made as to the need for increasing the cuff pressure level.

BACKGROUND OF THE INVENTION

The field of the invention is patient monitoring systems. Moreparticularly, the invention relates to a method and system that usesprevious blood pressure data to determine when to change the pressurestep size during blood pressure readings.

The heart muscles of humans periodically contract to force blood throughthe arteries. As a result of this pumping action, pressure pulses existin these arteries and cause them to cyclically change volume. Thebaseline pressure for these pulses is known as the diastolic pressureand the peak pressure for these pulses is known as the systolicpressure. A further pressure value, known as the “mean arterialpressure” (MAP), represents a time-weighted average of the bloodpressure. The systolic, MAP and diastolic values for a patient areuseful in monitoring the cardiovascular state of the patient, todiagnose a wide variety of pathological conditions, and treat disease.Therefore, it is a great advantage to a clinician to have an automaticdevice which can accurately, quickly and non-invasively estimate theseblood pressure values.

There are different techniques and devices for measuring one or more ofthese blood pressure values. One method in particular involves applyingan inflatable pressure cuff about the upper arm of a human and inflatingit above systolic pressure so as to restrict the flow of blood in thebrachial artery. The pressure is then slowly relieved while astethoscope is used on the distal portion of the artery to listen forpulsating sounds, known as Korotkoff sounds, that accompany thereestablishment of blood flow in the artery. As the pressure in the cuffis reduced further, the Korotkoff sounds eventually disappear. The cuffpressure at which the Korotkoff sounds first appear during deflation ofthe cuff is an indirect measure of the systolic pressure and thepressure at which these sounds disappear is an indirect measure of thediastolic pressure. This method of blood pressure detection is generallyknown as the auscultatory method.

Another method of measuring blood pressure is referred to as theoscillometric technique. This method of measuring blood pressureinvolves applying an inflatable cuff around an extremity of a patient'sbody, such as the patient's upper arm. The cuff is inflated to apressure above the patient's systolic pressure and then reduced overtime while a pressure sensor continues to measure the cuff pressure. Thesensitivity of the sensor is such that pressure fluctuations within thecuff resulting from the beats of the patient's heart may be detected.With each beat there is a resulting small change in the artery volumewhich is transferred to the inflated cuff causing slight pressurevariations within the cuff which are detected by the pressure sensor.The pressure sensor produces an electrical signal showing theincremental cuff pressure and a series of small periodic variationsassociated with the beats of a patient's heart. It has been found thatthese variations, called “complexes” or “oscillations,” have apeak-to-peak amplitude which is minimal for applied cuff pressures abovethe systolic pressure. As the cuff pressure is decreased, theoscillation size begins to monotonically grow and eventually reaches amaximum amplitude. After it reaches a maximum amplitude, the oscillationsize decreases monotonically as the cuff pressure continues to decrease.Physiologically, the cuff pressure at the maximum value approximates theMAP. In addition, the complex amplitudes of cuff pressures equivalent tothe systolic and diastolic pressures have a fixed relationship to thismaximum value. Thus, the oscillometric method is based on measurementsof detected complex amplitudes at various cuff pressures.

Blood pressure measuring devices operating according to theoscillometric method are used for detecting the peak-to-peak amplitudeof the pressure complexes at various applied cuff pressure levels. Theamplitudes of these complexes, as well as the applied cuff pressure, arestored together as the device automatically changes the cuff pressuresover a range of interest. These peak-to-peak complex amplitudes definean oscillometric “envelope” and are evaluated to find the maximum valueand its related cuff pressure, which is approximately equal to MAP. Acuff pressure below the MAP value which produces a peak-to-peak complexamplitude having a certain fixed relationship to the maximum value, isdesignated as the diastolic pressure. Likewise, a cuff pressure abovethe MAP value which results in complexes having an amplitude with acertain fixed relationship to that maximum value is designated as thesystolic pressure. The ratios of complex amplitude at systolic anddiastolic pressures to the maximum complex amplitude at MAP, areempirically derived and assume varying levels depending on thepreferences of those of ordinary skill in the art. Generally, theseratios are designated in the range of 40% to 80%.

One way to determine estimates of blood pressure is to computationallyfit a curve to the oscillometric envelope defined by the complexamplitude versus cuff pressure data points which are measured by a bloodpressure monitor during a determination. The fitted curve may then beused to compute an estimate of the MAP value, which is approximately atthe maximum value of the fitted curve and is therefore easily determinedby finding the point on the fitted curve at which the first derivativeequals zero. From this maximum value data point, the systolic anddiastolic pressures may be computed by finding fixed percentages of themaximum complex amplitude on the curve and using the associated cuffpressure levels as the systolic and diastolic estimates. In this manner,indirect estimates of the systolic, MAP and diastolic arterial pressuresmay be found and ultimately output by an oscillometric device. The curvefitting technique has the value of smoothing the envelope information sothat artifact variations are minimized and no single point dominates inthe calculation of blood pressure, thereby resulting in more accurateestimates.

Usually, when taking an oscillometric blood pressure determination, adevice will pump up to a supra-systolic cuff pressure level and takesmall deflation steps in order to completely measure the properties ofthe oscillometric envelope. However, pumping to higher than necessarycuff pressure levels and taking smaller than necessary steps may causepatient discomfort. Discomfort often results in patient motion whichincreases the likelihood of artifact, especially in pediatric andneonatal patients. Increased motion artifact may cause anon-determination or delay information output to the clinician.Therefore, to enhance patient comfort and reduce determination time, itis often desirable to take blood pressure readings with a minimal numberof pressure steps. The oscillometric envelope pattern is simple and byjudiciously choosing the particular cuff pressure levels to visit, thenumber of steps needed to compute an accurate blood pressure can besignificantly lowered. This involves making decisions about what cuffpressure levels to visit based on the measurements and results ofprevious blood pressure determinations. It is generally known in the artthat by visiting key points around the systolic, MAP and diastolicpressure levels, an accurate blood pressure can be estimated without theneed to fill out every characteristic of the oscillometric envelope.Thus, the initial cuff pressure and size of the steps with which todeflate the cuff can be optimized if a previous blood pressuredetermination has been made and the blood pressure has not changedsignificantly. This means that the deflation steps will be much biggerthan what would be used if the blood pressure were not known. Typically,these larger cuff pressure steps can be in the range of 12 to 20 mm Hgand are chosen so as to go to specific cuff pressure levels basedprimarily on the systolic, MAP, and diastolic pressure values of theprevious determination. Since it is necessary for a patient's bloodpressure to remain substantially similar to the previous determinationbefore the accelerated inflation and deflation scheduling can beundertaken, the algorithm must have a means of guaranteeing that theblood pressure has not significantly changed. In situations where theblood pressure has significantly changed or is changing, it becomesnecessary to return to a cuff deflation scheme in which the step sizesare smaller so that the details of the oscillometric envelope will becaptured. Very often, these smaller cuff pressure steps are in the rangeof 2 to 8 mm Hg. In these circumstances, the act of returning to adifferent cuff pressure deflation scheme with smaller cuff pressuresteps in order to obtain the full range and detail of oscillometricenvelope data is called a reversion. Oftentimes, it is difficult toquickly and accurately determine when a reversion should be made. Thus,there exists a need for a method and system for quickly and effectivelydetermining when to make a reversion to smaller cuff pressure steps andwhether to increase or decrease cuff pressure during the reversion basedon previous blood pressure determinations.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method of determiningwhen to make a reversion to smaller cuff pressure steps during anoscillometric blood pressure measurement. The method includes comparingconformance of oscillometric envelope blood pressure data with previousblood pressure data, including measuring a shift between theoscillometric envelope blood pressure data and an oscillometric envelopederived from the previous blood pressure data. In addition, the methodincludes making a reversion decision based on whether the shift exceedsan allowable threshold.

Another embodiment of the present invention provides a method ofdetermining when to make a reversion to smaller cuff pressure stepsduring an oscillometric blood pressure measurement comprising comparingconformance of oscillometric envelope blood pressure data with previousblood pressure data, which includes evaluating whether oscillometricenvelope amplitudes exceed an allowable tolerance from the previousblood pressure data. In addition, the method includes making a reversiondecision based on whether the oscillometric envelope amplitudes exceedthe allowable tolerance.

Another embodiment of the present invention provides a method ofdetermining when to make a reversion to smaller cuff pressure stepsduring an oscillometric blood pressure measurement including evaluatingwhether an oscillometric envelope acquisition process has been completedand making a reversion decision based on whether the envelopeacquisition process has been completed.

Another embodiment of the present invention provides an apparatus formeasuring blood pressure including an inflatable cuff, a pressurizingapparatus, a cuff pressure sensor, and a programmed control device. Thepressurizing apparatus is coupled to the cuff for selectively applyingpressure by inflating or deflating the cuff. The cuff pressure sensor iscoupled to the cuff for sensing cuff pressure and blood pressureoscillations. Further, the programmed control device controls thepressure cuff and pressurizing apparatus, compares conformance ofoscillometric envelope blood pressure data with previous blood pressuredata, including measuring a shift between the oscillometric envelopeblood pressure data and an oscillometric envelope derived from theprevious blood pressure data, and makes a reversion decision based onwhether the shift exceeds an allowable threshold.

Another embodiment of the present invention provides a system fordetermining when to make a reversion to smaller cuff pressure stepsduring oscillometric envelope blood pressure determinations including ameans for comparing conformance of oscillometric envelope blood pressuredata with previous blood pressure data, including measuring a shiftbetween the oscillometric envelope blood pressure data and anoscillometric envelope derived from the previous blood pressure data. Inaddition, the system includes a means for making a reversion decisionbased on whether the shift exceeds an allowable threshold.

Another embodiment of the present invention provides a computer programproduct comprising a computer useable medium having computer logic forenabling at least one processor in a computer system to determine whento make a reversion to smaller cuff pressure steps during oscillometricenvelope blood pressure determinations. In addition, the computerprogram product includes a means for comparing conformance ofoscillometric envelope blood pressure data with previous blood pressuredata, including measuring a shift between the oscillometric envelopeblood pressure data and an oscillometric envelope derived from theprevious blood pressure data. Furthermore, the computer program productincludes a means for making a reversion decision based on whether theshift exceeds an allowable threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a non-invasive blood pressure monitoring systemin accordance with an embodiment of the present invention.

FIG. 2 displays typical waveforms for a normal oscillometricnon-invasive blood pressure determination with amplitude ofoscillometric pulses shown as a function of time.

FIG. 3 is a summary flow chart showing a process for measuring bloodpressure according to an embodiment of the present invention.

FIG. 4A is a detailed flow chart showing a process for measuring bloodpressure according to an embodiment of the present invention.

FIG. 4B is a flow chart showing a process for measuring blood pressureusing curve fit information involving reverting and searching techniquesaccording to an embodiment of the present invention.

FIG. 5 illustrates a shift in an oscillometric envelope due to a changein blood pressure according to an embodiment of the present invention.

FIG. 6 illustrates cuff pressure and step size control during anoscillometric determination of blood pressure. This figure alsoillustrates a reversion step according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the arm of a human subject wearing a conventional flexibleinflatable and deflatable cuff 101 for occluding the brachial arterywhen fully inflated. As cuff 101 is deflated using deflate valve 102having exhaust 103, the arterial occlusion is gradually relieved. Thedeflation of cuff 101 via deflate valve 102 is controlled bymicroprocessor 107 via control line 116.

A pressure transducer 104 is coupled by a hose or duct 105 to the cuff101 for sensing the pressure therein. In accordance with conventionaloscillometric techniques and due to the compliant properties of theblood vessels, pressure oscillations in the artery caused by a heartbeat result in small cyclical volume changes in the artery. These smallvolume changes in the artery are transferred to the inflated cuffwrapped around the limb and finally result in small pressure changes inthe cuff 101. These cuff pressure oscillations are sensed by pressuretransducer 104 and converted into an electrical signal and coupled overpath 106 to microprocessor 107 for processing. In addition, a source ofpressurized air 109 is connected via a duct 110 through an inflate valve111 and a duct 112 to the pressure cuff 101. The inflate valve 111 iselectrically controlled through a connection 113 from the microprocessor107. Also, the deflate valve 102 is connected by duct 114 via a branchconnection 115 with the duct 112 leading to cuff 101. This deflate valve102 is normally closed during the inflation process and is brieflyopened by a deflate control mechanism to provide the pressure steplevels where cuff oscillations are detected.

FIG. 2 displays typical waveforms for a normal oscillometricnon-invasive blood pressure determination with amplitude ofoscillometric pulses gathered at different cuff pressures. Two waveformsare shown. Curve 121 represents the overall cuff pressure of theinflatable cuff and curve 123 represents the measured pulse amplitudesfor oscillometric complexes at various cuff pressures as time throughthe determination progresses. Curves 121 and 123 can be used toconstruct an oscillation amplitude versus cuff pressure curve which isknown as the oscillometric envelope. As can be seen, the cuff is firstinflated to a supra-systolic pressure 120, and then reduced in a seriesof small incremental steps, such as steps 122, 124, 126. Oscillations128 corresponding to each pulse are measured at each incremental cuffpressure. The peak pulse amplitudes (PPA) of each oscillation increasewith each decrement of cuff pressure until the PPA reaches a maximum atcuff pressure 124. The PPA diminishes with every subsequent reduction incuff pressure. Thus, the cuff pressure at step 124 represents thepatient's MAP, and the patient's systolic and diastolic pressures can bedetermined therefrom. Although FIG. 2 shows incremental decreases inpressure steps, similar determinations as those above may also be madefrom continuous or linear decreases in pressure over time rather thanincremental steps.

Referring to FIG. 3, a general flow chart representing a process fordetermining blood pressure is shown. Process 100 shows a method ofdetermining when to make a reversion to smaller cuff pressure stepsduring oscillometric blood pressure measurements based on the existenceof a previous blood pressure determination. First, measurements of bloodpressure complexes are obtained to determine the oscillometric envelopeat step 102. The measurements are initially obtained using relativelylarger pressure steps. At step 104, a comparison is made for theconformance of oscillometric envelope blood pressure data with previousblood pressure data. This includes measuring a shift between theoscillometric envelope blood pressure data and an oscillometric envelopederived from the previous blood pressure data. At step 106, a reversiondecision is made based on whether the shift exceeds an allowablethreshold. Finally, the blood pressure measurement is completed usingrelatively smaller steps and blood pressure is computed at step 108.Alternatively, a stop decision is made at step 108.

FIG. 4A is a detailed flow chart showing process 150 for determiningwhen to make a reversion decision during a blood pressure determination.The process begins at step 152 and pumps up the cuff to a specifiedinflation pressure for the particular patient as is generally known inthe art. As is usually the case, the first cuff pressure is set greaterthan the previous systolic pressure. Next, the cuff is deflated at step156 to a specified pressure. Oscillations are gathered at step 158 ifpossible. At step 160, it is determined whether the process times outwithout finding complexes. If the process times out, a reversion hastaken place and it is determined whether a pump up in pressure isrequired at step 164. If the pump up is not necessary, the processcontinues to step 168 and completes the blood pressure determinationusing small steps. If the pump up is necessary at step 164, a pump upoccurs at step 166 based on the oscillometric envelope data gathered gofar. Then the process goes to step 168. After step 168, the processwould end the determination and output the blood pressure.

Referring back to step 160 in FIG. 4A, if there was not a time out infinding complexes, the process proceeds to step 162. At step 162, thecomplexes are evaluated for a possible reversion. The criteria evaluatedfor a possible reversion includes: identifying the oscillometricenvelope shift, verifying that the complexes are within thepredetermined tolerance, evaluating the shape of the raw oscillometricenvelope, and verifying that there are enough points on either side ofthe oscillometric envelope. After step 162, the process determines if areversion should occur. If so, the process proceeds to step 164 asdescribed above. Otherwise, the process analyzes whether to end thedetermination at step 172. If more data is required, the process returnsto step 156. However, if there is enough data, the process proceeds tostep 174 and determines blood pressure. Finally, the process outputs theblood pressure at step 176.

Referring to FIG. 4B, a flow chart is provided that shows certain stepsand criteria for making a reversion decision during a blood pressuredetermination. The basic underlying assumption for process 200 is that aprior oscillometric envelope derived from previous measurements alreadyexists. The previous oscillometric envelope and its associated curvefitparameters are then used in evaluating the current oscillometric dataand to help determine whether a reversion needs to be made. For example,a method of employing curvefit parameters is described in U.S. Pat. No.5,704,362 to Hersh et al. The current oscillometric envelope data, if onthe same patient and within a reasonable time period, is expected tohave the same relationship between the oscillation amplitude and theassociated cuff pressure as the previous blood pressure determination.Therefore, new oscillation data points are expected to compare favorablyto those predicted by the curvefit from the previous determination ifthere has not been a significant change in blood pressure. This type ofcomparison occurs at step 202. An example of a shift in theoscillometric curve is shown in FIG. 5. The initial oscillation curve isindicated by curve 302 and the shifted curve is indicated by curve 304.The difference in cuff pressure between the two data sets, which is anindication of envelope shifting, can be measured from point 306 to point308. An envelope shift will result if there is a noticeable change inthe blood pressure of a patient. However, there has to be some tolerancein making the decision to revert since it is not necessary to do so ifthere are only small changes in the complex amplitudes unrelated toblood pressure. In situations, where blood pressure has changed, process200 provides mechanisms to ensure accurate blood pressure values. Themathematical technique for shifting the blood pressure envelope asmeasured in the previous determination to the blood pressure of thepresent determination uses the principle of aligning the previouslydetermined curve with the complex amplitudes measured during the currentdetermination. The shift that results after aligning the new data withthe curve from the previous determination is an estimate of how much theblood pressure has changed. This indication of change or shift can beused to decide, or at least influence, the decision of whether or not tomake a reversion. As this shift becomes large, it is less likely thatthe envelope characteristic will be measured well enough to accuratelydetermine the blood pressure. At this point, it is best for thecontrolling algorithm to cause a reversion (and pump up if necessary)and find the oscillation sizes using much smaller pressure steps.

Referring back to FIG. 4B, at step 202, the controller determineswhether the old envelope has to be shifted more than 10 mm of Hg tomatch the current data. In other words, an evaluation is made whetherthe difference between points 306, 308 on FIG. 5 is greater than a 10 mmHg threshold. Of course, the use of 10 mm Hg is merely exemplary and anynumber of other limits could also be used (e.g., 9 mm Hg, 11 mm Hg,etc.). If there has not been a shift of more than 10 mm Hg, the processdetermines whether the complexes in the current determination are withina tolerance of the last envelope at step 206. According to an embodimentof the present invention, this tolerance requires the oscillationamplitude obtained at a step in the current determination to be within+/−20% (20% is provided as an example) of the oscillation amplitude sizeas obtained from the curve fit of the last determination. Note also thatthe curve fit from the last determination may be shifted to handle smallchanges in blood pressure before use in this tolerance test.

Determining whether the complexes are within a tolerance providesanother test as to whether or not a significant physiological change hastaken place. Furthermore, a tolerance test based on how well new dataapproximates a previous curve fit may be an indication of the presenceof motion artifact which is not being adequately eliminated by otherparts of the algorithm. In such a situation, it might also be necessaryto cause a reversion and proceed with smaller deflation steps. Thus,evaluating how close the current pulse amplitudes are to the pulseamplitudes from a previous determination provides a powerful way to helpdecide if a reversion should be done. If the complexes are within anacceptable tolerance of the last envelope, the process determines atstep 212 whether the envelope acquisition process is complete. Thealgorithm must decide whether an adequate number of pressure steps havebeen visited for an accurate calculation of blood pressure. If theenvelope acquisition is not complete, the process proceeds to step 228and returns a “No” decision on whether to revert. In other words, theenvelope acquisition process would continue to obtain additional points.If the envelope acquisition is complete, the process makes adetermination at step 214 as to whether these data points are adequateto compute the blood pressure. Thus, if the algorithm finds that enoughpressure steps have been visited for a blood pressure determination, thefinal calculation of blood pressure can be made. Generally, to fill outthe oscillometric envelope, complexes are measured at cuff pressuresteps above systolic, below systolic but above MAP, below MAP but abovediastolic and below diastolic. Thus, there should be at least fourpressure steps to form an envelope. The data at these four pressuresteps, however, can be augmented by data from prior determinations oraugmented by predicted amplitudes at specific pressures derived from aprior curvefit. If there are enough points to adequately specify theoscillometric envelope, the process determines whether these points spanan appropriate amplitude range on the systolic side of the envelope atstep 216 and whether these points span an appropriate amplitude range onthe diastolic side at step 220. Typically, there should be at least twosteps on either side of the maximum of the oscillometric envelope curve.However, if there are only four points in the current determination, thetwo points on either side of the maximum can be inclusive of the maximumpoint. If there are enough points on the diastolic and systolic sides,then the process returns a “No” decision on whether to revert at step228.

When deciding to make a reversion, it is often necessary to make surethe shift in blood pressure is consistent from step to step. Thisrequirement places a multi-step requirement on recognizing the shiftbefore actually reverting. Returning to step 202, if the old envelopeshifted more than 10 mm Hg, the process determines at step 204 whetherthe shift has been consistently present. In other words, a shift greaterthan 10 mm Hg should be present for at least two steps before triggeringa revert. A repeated and consistent shift in the current determinationenvelope data is a strong indication of a change in blood pressure. Ifthere has been a consistent shift, the process determines whether theconsistent shift is positive at step 208. If the shift is positive, areversion is required with a pump up to a higher cuff pressure than wasfirst used during the current determination. If the consistent shift atstep 204 is negative, a reversion is done without a subsequent pump upat step 230. A similar decision is made after gathering what is expectedto be a complete envelope. If it is determined that there is not enoughdiastolic data at step 220, a reversion is necessary, but the need for apump up at this point is based on the characteristics of the envelopedata. The process determines whether the lowest complex size on thediastolic side is less than a predetermined limit (e.g., 50%) of themaximum complex size at step 222. The term “lowest” here refers to thelowest cuff pressure step used in the process up to that point. If thelowest complex size on the diastolic side is less than 50% of themaximum complex size, a determination is made at step 224 to revert at apressure below the lowest pressure used so far. This may not require apump up if the cuff pressure step is the lowest that has been used sofar in the process. However, if the lowest complex size on the diastolicside is not less than 50% of the maximum complex size, a decision ismade at step 226 to revert with a pump up to a pressure above the lowestcuff pressure step visited so far in the process. As one skilled in theart would appreciate, any number of other limits could also be used(i.e., 45%, 55%, etc.). Further, the envelope shift value may befiltered before being compared to the threshold to cause the reversion.This action would make sure that the change in blood pressure is worthresponding to with a reversion.

The details of the mathematical process of shifting an oscillometriccurve will now be addressed. Although many different forms can be used,an envelope curve fit may be represented according to the followingequation:Ai=Ae ^(−((Pi−B)) ² ^(/C)) =f(Pi; A,B,C)where A, B and C represent parameters which set the amplitude, the mean,and the spread of the envelope. A_(i) and P_(i) are the oscillationamplitude and cuff pressure, respectively, of a specific envelope datapoint. We compute and store the A, B and C parameters from theoscillometric envelope data of a prior blood pressure determination forfuture use. The envelope shift can be found by taking a new measuredpoint in the current determination (P_(i), A_(i)) for a given A and C(from previous determination) and inverting the formula to find B_(new).The amount of difference between the B_(new) and the original B is anestimate of the blood pressure shift. The actual inversion anddifference formula would then be:Bnew−B=Pi ±√{square root over ((Cln(A/Ai))}−B=Envelope Shift

Note that this is just an example for a Gaussian form envelope equation.The plus or minus could be determined by finding which one provides theleast shift. If the shift is so large that this technique doesn't causea reversion when one is necessary, the amplitude tolerance or shapecriteria described earlier will eventually cause the reversion. Also,the techniques for inversion to compute the shift could include anynumber of algorithms. An example might be Newton's method or a morebrute force search. Though the described techniques assume a single C(spread) parameter, one could use a different C (spread) parameter oneither side of the maximum on the oscillometric envelope as described inU.S. Pat. No. 5704362 to Hersh et al. In this event, the shiftcalculations would incorporate using different C (spread) parametersdepending on the location of the pressure step in relation to theoscillometric envelope. An example of the computation of the normalizedtolerance may be represented according to the following equation:Tolerance=|(Ai−f(Pi; A, B, C)/f(P i; A, B, C)|where A, B and C parameters are from the oscillometric envelope data ofa prior blood pressure determination, Ai is the oscillation amplitudebeing checked, and Pi is its corresponding cuff pressure.

While this equation provides one example for finding a tolerance value,those skilled in the art will realize that a number of other equationsor methods could be used. For example, rather than using the curve fitfrom a previous determination, tolerance could be based on the size ofthe oscillation amplitude at the closest step available in the lastdetermination or on the maximum oscillation size from the lastdetermination. The tolerance equation above should only be taken asexemplary.

As described herein, if there has not been a shift in the envelope dataof more than 10 mm Hg at step 202, the process determines whether thecomplexes are within a tolerance of the last envelope at step 206. Ifthe complexes are not within an acceptable tolerance, the processdetermines whether the complexes are consistently and repeatedly out oftolerance at step 210. Typically, for a complex to be consistently andrepeatedly out of tolerance, it must exceed the tolerance for two ormore steps. In addition, the allowable tolerance can vary depending onlocation along the oscillometric envelope (i.e., systolic, diastolic,MAP, etc.). If the complex exceeds the tolerance repeatedly, the processdetermines at step 218 that a reversion is necessary with a pump up tohigher cuff pressure. A higher cuff pressure typically means an increasein cuff pressure of about 40 mm Hg above the highest cuff pressure takenin the on-going determination. Of course, 40 mm Hg is merely exemplaryand any number of other measurements could also be used (i.e., 35 mm Hg,45 mm Hg, etc.).

FIG. 6 shows oscillometric data from a blood pressure measurement when areversion step is made during the process. Curve 270 represents theoverall cuff pressure of the inflatable cuff and curves 272, 274represent the measured peak pulse amplitudes for oscillometriccomplexes. The cuff pressure is increased as indicated by the upwardcurvature of line 248 as the cuff is inflated. Once a chosen pressure isselected at point 249, large deflate steps are initiated, as shown bystep 250. After several large steps, it is determined that the measuredpeak pulse amplitudes for the oscillometric complexes 254 areinappropriate. As is known in the art, when a cuff is inflated to apressure above a patient's systolic pressure and then incrementallyreduced in a series of small steps, the oscillations should begin smalland then gradually increase to a maximum. Since the oscillations 254began at a higher level and then decreased, the determination to revertoccurs at point 252. The reversion occurs and the cuff is once againinflated as shown by the upward curvature of line 266. Once a desiredpressure is attained, smaller incremental deflate steps 258, 260, 262,264 begin. The measured peak pulse amplitudes of oscillations 256 moreclosely follow the expected pattern of blood pressure oscillationamplitudes so that the full range of oscillation measurements areobtained. According to a preferred embodiment, oscillometric data is notpurged when doing the reversion. Instead, the new data is filled in tocomplete the oscillometric envelope.

While the embodiments and application of the invention illustrated inthe figures and described above are presently preferred, it should beunderstood that these embodiments are offered by way of example only.Accordingly, the present invention is not limited to a particularembodiment, but extends to various modifications that nevertheless fallwithin the scope of this application.

1-17. (canceled)
 18. A method of determining when to make a reversion tosmaller cuff pressure steps during an oscillometric blood pressuremeasurement, comprising: comparing conformance of oscillometric envelopeblood pressure data with previous blood pressure data, includingevaluating whether oscillometric envelope amplitudes exceed an allowabletolerance from the previous blood pressure data; and making a reversiondecision based on whether the oscillometric envelope amplitudes exceedthe allowable tolerance.
 19. The method of claim 18, wherein making areversion decision further includes evaluating whether the oscillometricenvelope amplitudes have consistently exceeded the allowable tolerance.20. The method of claim 18, further comprising evaluating whether anoscillometric envelope acquisition process has been completed, whereinmaking a reversion decision further includes determining whether theenvelope acquisition process has been completed.
 21. The method of claim20, further comprising evaluating whether a sufficient number ofoscillometric envelope data points have been obtained on systolic anddiastolic sides of the oscillometric envelope blood pressure data. 22.The method of claim 20, wherein making a reversion decision furtherincludes evaluating whether a lowest complex side on a diastolic side ofthe oscillometric envelope blood pressure data exceeds an allowablethreshold in order to decide whether a pressure change should be made.23. The method of claim 22, wherein the allowable threshold is about 50%of a maximum complex size in the oscillometric envelope blood pressuredata. 24-32. (canceled)
 33. The method of claim 18, further comprising:measuring a shift between the oscillometric envelope blood pressure dataand an oscillometric envelope derived from the previous blood pressuredata; and making a reversion decision based on whether the shift exceedsan allowable threshold.
 34. The method of claim 33, wherein making areversion decision further includes evaluating whether the shift hasbeen consistently present in the oscillometric envelope blood pressuredata.
 35. The method of claim 33, wherein making a reversion decisionfurther includes evaluating whether the shift is positive or negative.36. The method of claim 35, wherein making a reversion decision furtherincludes evaluating whether a lowest complex size on a diastolic side ofthe oscillometric envelope blood pressure data exceeds an allowablethreshold.
 37. The method of claim 36, wherein the allowable thresholdis about 50% of a maximum complex size in the oscillometric envelopeblood pressure data.
 38. The method of claim 18, further comprising:evaluating whether an oscillometric envelope acquisition process hasbeen completed; and making a reversion decision based on whether theenvelope acquisition process has been completed.
 39. The method of claim38, further comprising evaluating whether a sufficient number ofoscillometric envelope data points have been obtained.
 40. The method ofclaim 38, wherein making a reversion decision further includesevaluating whether a lowest complex side on a diastolic side of theoscillometric envelope blood pressure data exceeds an allowablethreshold.
 41. The method of claim 40, wherein the allowable thresholdis about 50% of a maximum complex side in the oscillometric envelopeblood pressure data.